Gyroscopes are sensors mounted in a sensor package, which are used to sense rotations of the platforms to which they are attached. When the sensor package that is attached to the platform is rotated, the gyroscope gives an output which is proportional to the angular velocity.
By way of background, sensing of angular velocity is frequently performed using vibratory rate gyroscopes. Vibratory rate gyroscopes broadly function by driving the sensor into a first motion and measuring a second motion of the sensor that is responsive to both the first motion and the angular velocity to be sensed.
Frequently, a mass, usually referred to as a proof mass, within the sensor is driven into oscillation by an actuator. Rotation of the sensor, imparts a Coriolis force to the oscillating mass that is proportional to the angular velocity (or rotation rate), and Coriolis force depends on the orientation of the angular velocity vector with respect to the velocity vector of the proof mass. The Coriolis force is perpendicular to the angular velocity vector, and the proof-mass velocity vector. The governing equation for the Coriolis force vector is represented as follows:{right arrow over (F)}Coriolis=2m{right arrow over (V)}d×{right arrow over (Ω)}  Eq. (1)
where, {right arrow over (F)}Coriolis is the Coriolis force imparted to the structure (or proof mass), m is the mass of the proof mass, {right arrow over (V)}d is the velocity of the proof mass in the drive direction, {right arrow over (Ω)} is angular velocity input. For example, a proof mass moving in an x-direction within a sensor rotating about the y-axis experiences a z-axis-directed Coriolis force. Similarly, a proof mass moving in the x-direction within a sensor rotating about a z-axis experiences a y-axis-directed Coriolis force. Finally, a proof-mass moving in the x-direction within a sensor rotating about the x-axis experiences no Coriolis force. Coriolis forces imparted to the proof-mass are usually sensed indirectly by measuring motions within the sensor that are responsive to the Coriolis forces.
If it is desirable to test the gyroscope in terms of its functionality, the sensor package can be rotated and the output of the gyroscope is monitored to determine if the gyroscope is responding to the given input rotation. A drawback of moving the sensor package to test the functionality of the gyroscope is the need for costly testing platforms and the large amount of time consumed to do the testing.
On the other hand, there are also other ways to test gyroscope functionality without moving the sensor package. One of the methods that is widely used to test the gyroscope functionality is called self-test. Self-test in gyroscopes is defined as the testing of the gyroscope functionality without physically rotating the sensor package.
Conventional gyroscopes comprise two main systems. One of the systems is called the drive system, and the second system is called the sense system. In a conventional gyroscope, the output response to the given rotation input depends on both the drive and sense systems. In order to test full functionality of the gyroscope, both systems have to be tested for functionality in the self-test operation.
More specifically, in currently-implemented self-test methods, there are two primary approaches for testing the sensor functionality. A first method includes an actuation mechanism that vibrates the proof mass of the gyroscope in a direction along the responsive axis (also known as the “sense axis”). In this method, the proof mass of the gyroscope is vibrated with a known input force in the sensitive direction, and the output of the sense system is monitored to determine if the output response is at a desired level. With this method, demonstrating the drive system functionality requires additional testing to be applied on the drive system mechanisms. As a result, sense and drive systems cannot be tested simultaneously, and this requires extra time and effort to complete full functionality testing of the entire gyroscope.
A second method includes an actuation mechanism that can rotate the proof mass of the gyroscope along the sensitive axis (or “direction”). In this method, a known input rotation is applied to the proof mass at the chip level at a low frequency range. The low frequency range can be in the range of a couple of Hertz (Hz) to 500 Hz depending on the bandwidth of the gyroscope. In this second method, the proof mass is usually attached to a frame structure that supports the entire gyroscope and the frame structure is used to rotate the entire proof mass around the sensitive axis. With this method, however, because the self-test actuation is completed at low frequencies, the frame structure has to be particularly compliant to move large angles at sufficiently high angular rates in order to get a detectable self-test output. Compliant frame structures make the sensor more susceptible to unwanted external effects, like package stresses and vibrations, which can adversely affect the regular operation of the gyroscope. Moreover, due to the high compliance of the frame structure, the gyroscope is undesirably less stable against external mechanical shocks.
There is thus a need for a method and apparatus for reliably testing the full functionality of the gyroscope while maintaining the stability against unwanted external disturbances that may affect its regular operation.